Engineers achieve longlasting goal of stable nanocrystalline metals

Most metals are made of crystals: orderly arrays of
molecules forming a perfectly repeating pattern. In many cases, the material is
made of tiny crystals packed closely together, rather than one large crystal.
Indeed, for many purposes, making the crystals as small as possible provides
significant advantages in performance, but such materials are often unstable:
The crystals tend to merge and grow larger if subjected to heat or stress.

Now, Massachusetts Institute of Technology (MIT) researchers
have found a way to avoid that problem. They've designed and made alloys that
form extremely tiny grains—called nanocrystals—that are only a few billionths
of a meter across. These alloys retain their nanocrystalline structure even in
the face of high heat. Such materials hold great promise for high-strength
structural materials, among other potential uses.

The new findings, including both a theoretical basis for
identifying specific alloys that can form nanocrystalline structures and
details on the actual fabrication and testing of one such material, are described
in Science.

Graduate student Tongjai Chookajorn, of MIT's Department of
Materials Science and Engineering (DMSE), guided the effort to design and
synthesize a new class of tungsten alloys with stable nanocrystalline
structures. Her fellow DMSE graduate student, Heather Murdoch, came up with the
theoretical method for finding suitable combinations of metals and the
proportions of each that would yield stable alloys. Chookajorn then
successfully synthesized the material and demonstrated that it does, in fact,
have the stability and properties that Murdoch's theory predicted. They, along
with their advisor Christopher Schuh, the Danae and Vasilis Salapatas Professor
of Metallurgy and department head of DMSE, are co-authors of the paper.

For decades, researchers and the metals industry have tried
to create alloys with ever-smaller crystalline grains, Schuh says. But, he
adds, "nature does not like to do that. Nature tends to find low-energy states,
and bigger crystals usually have lower energy."

Looking for pairings with the potential to form stable
nanocrystals, Murdoch studied many combinations of metals that are not found
together naturally and have not been produced in the laboratory. "The
conventional metallurgical approach to designing an alloy doesn't think about
grain boundaries," Schuh explains, but rather focuses on whether the different
metals can be made to mix together or not. But, he adds, it's the grain
boundaries that are crucial for creating stable nanocrystals. So Murdoch came
up with a way of incorporating these grain boundary conditions into the team’s
calculations.

Why go to the trouble of designing such materials? Because
they can have properties that other, more conventional metals and alloys do
not, the researchers say. For example, the alloy of tungsten and titanium that
the MIT researchers developed and tested in this study is likely exceptionally
strong, and could find applications in protection from impacts, guarding
industrial or military machinery or for use in vehicular or personal armor. But
the researchers stress that this fundamental research could lead to a wide
range of potential uses. "This is one case study, but there are potentially
hundreds of alloys we could make," Schuh says.

Other nanocrystalline materials designed using these methods
could have additional important qualities, such as exceptional resistance to
corrosion, the team says. But finding materials that will remain stable with
such tiny crystal grains, out of the nearly infinite number of possible
combinations and proportions of the dozens of metallic elements, would be
nearly impossible through trial and error. "We can calculate, for hundreds of
alloys, which ones work, and which don't," Murdoch says.

The key to designing nanocrystalline alloys, they found, is "finding the systems where, when you add an alloying element, it goes to the
grain boundaries and stabilizes them," Schuh says, rather than distributing
uniformly through the material. Under classical metallurgical theory, such a
selective arrangement of materials is not expected to occur.

The tungsten-titanium material that Chookajorn synthesized,
which has grains just 20 nm across, remained stable for a full week at a temperature
of 1,100 C—a temperature consistent with processing techniques such as
sintering, where powdered material is packed into a mold and heated to produce
a solid shape. This means this alloy could easily become a practical material
for a variety of applications where its high strength and impact resistance
would be important, the researchers say.